There are many examples of communications systems in which data is communicated using Orthogonal Frequency Division Multiplexing (OFDM). Systems which have been arranged to operate in accordance with Digital Video Broadcasting (DVB) standards for example, utilise OFDM. OFDM can be generally described as providing K narrow band sub-carriers (where K is an integer) which are modulated in parallel, each sub-carrier communicating a modulated data symbol such as Quadrature Amplitude Modulated (QAM) symbol or Quadrature Phase-shift Keying (QPSK) symbol. The modulation of the sub-carriers is formed in the frequency domain and transformed into the time domain for transmission. The sub-carriers are modulated in parallel contemporaneously, so that in combination the modulated carriers form an OFDM symbol. The OFDM symbol therefore comprises a plurality of sub-carriers each of which has been modulated contemporaneously with different modulation symbols.
In order to allow data to be recovered from the OFDM symbols in the presence of multi-path which causes the same OFDM symbols to be received via echo paths and/or simulcast in which the same OFDM symbols are received from more than one transmitter, it is conventional to include a time domain guard interval between successive OFDM symbols. The guard interval is formed by repeating samples in the time domain from a ‘useful’ part of the OFDM symbols. The useful part of the OFDM symbols correspond to the samples in the time domain which are formed when the modulated sub-carriers are transformed into the time domain from the frequency domain. As a result of the guard interval, all of the samples from the useful part of the OFDM symbols can be received by a receiver provided that the multi-path or the simulcast delay between versions of the same OFDM symbols does not exceed the guard interval.
However, detecting and recovering data from the useful part of the OFDM symbols at the receiver can nevertheless present a technical problem.